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107

Trachytes and their feldspar phenocrystsBy I . S . E . CARMICRAEL

D e p a r t m e n t o f G e o l o g y , I m p e r i a l C o l le g e o f S c ie n c e, L o n d o n , S . W . 7

S u m m a r y . The compositions of the groundmass and of the plagioclase andsanidine phenocrysts o f a series of over- and und er-saturated trachytes indicateth at plagioclase will precipitate at some stage in the crystallization interval of allcalcium-bearing potassic trachytes. Using this d ata obtained from natura l salicliquids, a concep t o f crystallization within the system CaA12Si~Os-NaA1SiO -KA1SiO4-SiO2 m ay be obtained.

L T H O U G H t h e s y s t e m C a A I 2S i2 O s -N a A 1 S iO 4 - K A 1 S i0 4 -S iO2 h a ss t il l t o b e i n v e s t i g a t e d e x p e r i m e n t a l l y , i t is p o s si b le t o d e d u c e s o m e

o f t h e p r o p e r t ie s o f th i s s y s t e m b y t h e s t u d y o f t h e p h a s e c h e m i s t r y o fn a t u r a l r o c k s w h o s e b u l k c o m p o s i t i o n m a y b e e x p r e s s e d a l m o s t c o m -p l e t e l y i n t e r m s o f t h e s e f o u r c o m p o n e n t s . A s a re s u l t o f a s t u d y o f t h ep o r p h y r i t i c r h y o l i t e s ( C a r m i ch a e l, 1 9 63 a ) w h i c h p l o t i n t h e o v e r - s a t u r a t e dp a r t o f t h e f o u r - c o m p o n e n t sy s t em , i t w a s c o n c lu d e d t h a t p o t a ss ic a c idl i q u i d s w o u l d , a t s o m e s t a g e i n t h e i r c r y s t a l l i z a t i o n i n t e r v a l , i n t e r s e c t ab o u n d a r y s u r f a c e , a n d t h a t o n t h i s s u r f a c e t w o f e l d s p a r s , a p l a g i o c l a s e ,a n d a s a n i d i n e w o u l d p r e c i p i t a t e .

T h i s p a p e r i s c o n c e r n e d w i t h t h e t w o - f e l d s p a r p o t a s si e t r a c h y t e s , a n dt h e r e f o r e e x t e n d s t h e i n v e s t i g a t e d c o m p o s i t io n s o f n a t u r a l s alic li q u id si n t h e s y s t e m C a A I ~ S i ~ O s - N a A I S i O ~ - K A I S i 0 4 - S i O2 f r o m t h e q u a r t z -f e l d s p a r b o u n d a r y s u r f a c e ( r h y o l i t e s ) (op . c i t . ) a c r o s s t h e t e r n a r yf e l d s p a r p l a n e t o w a r d s t h e p h o n o l i t e m i n i m u m ( H a m i l t o n a n d M a c -K e n z i e , 1 9 65 ). S o m e o f t h e a r g u m e n t s a n d d i sc u s s io n d e v e l o p e d i n ap r e v i o u s p a p e r ( C a r m ic h a e l, 1 9 6 3 a) h a v e b e e n a b b r e v i a t e d o r o m i t t e dt o a v o i d r e p e t i ti o n .

PetrographyT h e p e t r o g r a p h i c d e s c r i p t io n s o f t h e t r a c h y t e s a r e g i v e n o il p a g e 1 24

a n d a r e s u m m a r i z e d b e lo w . I n t h e d i s cu s s io n w h i c h f o ll o w s i t is c o n -v e n i e n t t o d i v id e t h e t r a c h y t e s i n t o t w o t y p e s : t h o se w h i c h ar eo v e r s a t u r a t e d a n d c o n t a i n n o r m a t i v e q u a r t z , a n d t h o s e w h i c h a r eu n d e r s a t u r a t e d a n d c o n t a in n o r m a t i v e ne p h el in e .

P h e n o c r y s t s o f p l ag i o cl as e a n d s a n i d in e a r e f o u n d i n a ll th e r o c k s

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108 I . S. E. CARM ICHAEL ONe x c e p t 8 R 1 ; p l a g i o c la s e is g e n e r a l l y t h e f i rs t f e l d s p a r t o p r e c i p i t a t e , a sp l a g io c l a se c r y s t a l s a r e s o m e t i m e s e n c l o s e d b y t h e s a n i d in e p h e n o c r y s t s .W i t h t h e e x c e p t i o n o f t h e f e l d s p a r s i n 5 R , n e i t h e r t h e p l ag i o cl as e n o r t h es a n i d i n e i s s t r o n g l y z o n e d , b u t a s p la g i o c la s e is n o r m a l l y t h e f ir s t f e l d s p a rt o s e p a r a t e , p o s s i b ly o n l y t h e o u t e r z o n e s o f t h e c r y s t a l s a r e i n e q u il i-b r i u m w i t h t h e s a n i d in e p h e n o c r y s t s . T h i s z o n i n g h a s b e e n n e g l e c te d ,a l t h o u g h i n g e n e r a l t h e c o m p o s i t i o n o f t h e p l a g io c l a se p h e n o c r y s t s a sf o u n d b y a n a l y s i s w i ll b e s l i g h t ly m o r e c a lc ie t h a n t h e i r a c t u a l c o m p o s i -t io n , a s th e o u t e r r i m s o f c r y s t a l s t e n d t o b e l o s t i n s e p a r a ti o n .

Q u a r t z c a n b e i d e n t if i e d i n th e g r o u n d m a s s o f t h e m o r e c o a r s e l yc r y s t a ll i n e o v e r s a t u r a t e d t r a c h y t e s , b u t is n e v e r f o u n d a s a p h e n o c r y s t .I n t h e g r o u n d m a s s e s o f t h e u n d e r - s a t u r a t e d r o c k s, a n in t e r s ti ti a l f el s-p a t h o i d , p r o b a b l y s o d a li te , i s p r e s e n t , b u t a f e l d s p a t h o i d is n e v e r p r e s e n ta s a p h e n o c r y s t .

B i o t i te i s a p p a r e n t l y s t a b l e i n o n l y o n e o f t h e t r a e h y t e s ( 1R ) , a n d i nt h e r e m a i n i n g b i o t it e - b e a r i n g s p e c im e n s it h a s b e e n r e s o r b e d a n d r e -p l a c e d b y a g g r e g a t e s o f g r a n u l a r i r o n -o r e t o a g r e a t e r o r le ss e r e x t e n t .B e c a u s e b i o t i t e s c o n t a i n a s i g n i f ic a n t p r o p o r t i o n o f a l k a l i f e l d s p a rc o m p o n e n t s , t h e i n c o r p o r a t i o n o f t h es e c o m p o n e n t s i n t o t h e l iq u id m a yc h a n g e t h e c o m p o s i t io n o f t h e l i q u i d 's f e l d s p a t h i c c o m p o n e n t s . A s t h ei n t r a t e l l u r i c c r y s t a l l i z a t i o n o f b i o t i t e in t h e s e t r a c h y t e s i s t a k e n t o b eb r o a d l y c o e v a l w i t h t h e c r y s t a l li z a t i o n o f t h e f e l d s p a r p h e n o c r y s t s , t h es u b s e q u e n t r e s o r p t i o n o f t h e b i o ti te , d u e p o s s i b l y t o r e le a se o f w a t e rv a p o u r p r e ss u r e, w i ll n o t g r e a t l y a f f e c t t h e c o m p o s i t i o n o f t h e f e l d s p a rp h e n o c r y s t s . M o r e o v e r , a s t h e m o d a l a m o u n t o f b i o t it e is u s u a l l y le sst h a n 2 "5 % ( ta b l e I ) t h e i n c o r p o r a t i o n o f i ts f e l d s p a th i c c o m p o n e n t si n t o a l iq u i d c o m p o s e d o f o v e r 7 0 % a l k al i f e l d s p a r c o m p o n e n t s w i ll n o tg r e a t l y c h a n g e t h e l i q u i d ' s c o m p o s i t i o n .

A c o m m o n f e a t u r e i n th e s e t r a e h y t e s is t h e a l m o s t u b i q u i t o u s p r e s e n c eo f m i e r o p h e n o c r y s t s o f i r o n - o r e e n c l o se d i n a u g i t e p h e n o c r y s t s ; i n t h em o r e i r o n - r i c h f e r r o a u g i t e p h e n o c r y s t s t h i s o c c u r r e n c e i s l e s s n o t i c e a b l e( C a r m i e h a e l 1 96 3b ). A li m e - p o o r p y r o x e n e , e i t h e r p i g e o n i t e o r o r t h o -p y r o x e n e , h a s n o t b e e n f o u n d .

T h e r e i s n o p e t r o g r a p h i c e v i d e n c e t h a t a n y o f t h e p h e n o c r y s t s a r ex e n o c r y s t s ; t h e r e s o r p t i o n o f b i o t it e ( a n d a m p h i b o l e i n 6 R ) b y m a n y o ft h e s e t r a c h y t i c l iq u i d s is n o t e v i d e n c e o f t h e i r f o r e i g n o r i g in a n d n o

1 In the descriptions, the suffix R has been used for the rock, G denotes thegroundmass and A and B are th e alkali feldspar and plagioelase phenocrysts re-spectively. Thus 1A and 1B are the feldspar phenocrysts w hich together w ith th egroundmass 1G (and the ferromagnesian phenocrysts) make up the rock 1R.

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T R A C H Y T E S A N D T H E I R F E L D S P A R P H E N O C R Y S T S 1 0 9x e n o l i th s h a v e b e e n f o u n d . I n b o t h g r o u p s o f t r a c h y t e s i t is a s su m e dt h a t t h e r o c k s a re n o n - a c c u m u l a t i v e , o r i n o t h e r w o r ds , t h a t t h e y a r e a llc a p a b l e o f e x i s t in g i n a c o m p l e t e l y l iq u i d s t a t e ( a n d a t so m e s t a g e o fthe i r h i s to ry d id so ) in a na tu r a l e nv i ro nm ent . I t i s poss ib le to f inda n a l y se s o f f i n e - g r a i n e d a l m o s t c o m p l e t e l y n o n - p o r p h y r i t i c t r a e h y t i c

T AB LE I . M o d a l a n a l y s e s o f t r a c h y t e s ( v o l u m e p e r c e n t . )G r o u n d - I r o nm a s s P l a g io e l a s e S a n i d i n e B i o t i te P y r o x e n e O re M i s c e l la n e o u s

1 R 8 4 . 0 1 2 . 0 0 . 2 1 .1 0 . 8 0 . 6 t 1 .3 a m p h i b o l et t r . s p h e n e21% 7 1 . 4 2 4 . 5 0 . 9 * 1 . 4 0 . 6 0 . 9 0 . 3 s p h e n e31% 6 4 . 6 2 3 . 5 6 . 8 2 . 2 1 . 8 0 . 8 0 . 3 s p h e n e41% 86 .3 0 .8 12 .0 t r . 0 .2 0 .7 - -5 R 9 1 . 2 1 .1 5 . 2 1 -3 0 . 7 0 . 4 0 . 1 o l i v i n e61% 8 3 .8 4 .7 4 .0 5.1"~ 1 .0 1.1 { 0.10"3olivineSphene7 R 7 1 .9 t r . 2 7 .8 - - 0 . 3 t r. t r . s p h e n e8 R - - t r . 9 2 .0 - - 1 "8 1 .5 R e s t 4 ' 7

101% 79-3 t r . 19 .2 - - 1 -0 0 .5 - -* U n d e r e s t i m a t e d a s th e s a n i d i n e p h c n o c r y s t s a r e e x t re m e l y l a rg e ."~ B i o t i t e a n d b a r k e v i k i t e i n a l l s ta g e s o f d i s s o l u t i o n .

1 R . P o r p h y r i t i c t r a c h y t e p i t c h s t o n e , M u r a t , C a n t a l , F r a n c e . 3 8 11 .2 R . P o r p h y r i t i c tr a c h y t e , D r a c h e n f e l s, R h i n e , G e r m a n y . 3 11 1 a n d 1 3 9 5.3 R . P o r p h y r i t i c t r a c h y t e , S i e b e n g eb i rg e , G e r m a n y . B M . 1 9 0 9- 2 6 1 .4 R . C r i s t o b a l i t e - tr a c h y t e , J a n M a y e n , 5 T or th A t l a n t i c . J M - 1 2 2 .5 R . B i o t i t e - t r a c h y t e d y k e , G o u g h I s l a n d , S o u t h A t l a n t i c . G - 2 4.6 R . B i o t i t e - t r a c h y t e , T r i s t a n d a C u n h a . 4 4 2.7 R . T r a c h y t e , T r i s t a n d a C u n h a . 4 3 9.8 R . S a n i d i n i t e , E i fe l , G e r m a n y . 5 6 5 3.

1 0 R . P o r p h y r i t i c p i t e h s t o n e , I s l e o f E i g g , S c o t l a n d . E C . 2 0.l a v a s w h i c h m a t c h t h e a n a l y se s o f t h e t r a c h y t e s d e sc r i b ed h e r e. Th er o c k a n a l y se s a r e t a k e n t o r e p r e se n t t h e c o m p o s i t i o n o f i n it i al l iq u i ds .

T h e m o d a l a n a l y s es o f b o t h t y p e s o f t r a c h y t e s a r e g i v e n i n t a b l e I ,a n d t h e c h e m i ca l a n a ly s e s a n d C I P W n o r m s o f t h e o v e r s a t u r a t e dt r a e h y t e s a n d t h e ir g ro u n d m a s s e s a r e p r e s e n t e d i n t a b le I I . T h e d a t af o r t h e u n d e r s a t u r a t e d t r a c h y t e s a r e g i v e n i n t a b l e I I I , a n d t h e lo c a li ti e so f a l l t h e sp e c i m e n s a r e g i v e n b e l o w t a b l e I .

A l l o f t h e se t r a e h y t e s a n d t h e s a n i d i n i t e 8 R h a v e p r e c i p i t a t e d ap l a g io c l a se t o g e t h e r w i t h s an i d in e , a n d t h e p u r p o se o f t h i s p a p e r i s t osh o w t h a t t h e r e so r p t i o n o f p l a gi o c la se d e p i c t e d so c l e a r l y i n 4 R , 7 R ,a n d 8 R i s t h e r e su l t o f a re a c t i o n r e l a t io n sh i p c o m m o n t o a ll n o r m a t i v ea n o r t h i t e - b e a r i n g p o t a s s i c t r a c h y t e s , a n d w h i c h u n d e r e q u i l i b r i u m c o n -d i t i o n s , m a y b e t a k e n t o c o m p l e t i o n .

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110 I . S . E . C A R ~ I I C ~ A E [ ~ O N

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TRACHYTES AND THEIR FELDSPAR PHENOCRYSTS 111The anal yses of the feldspar pheno cry sts are given in table IV ; in each

case, the puri ty of the feldspar concentrate was careful ly checked inrefractive index oils before analysis, and each feldspar is believed to beat least 9S% pure.

TABLE III. Analyses and norms of undersa tura ted trachytes(For the key to analyses, see page 109)5R 5G 61% 6G 7R 81~

SiO 2 60.09 60 .8 0 56 .3 2 59.35 62 -2 7 62.06TiO~ 0.93 0.84 1.70 0.97 0.48 0-50AI~O~ 17 .9 5 17. 40 18-11 18 -7 3 1 9.4 7 18.99:Fe~O~ 1.39 1.74 2.02 1.47 1-44 1.59FeO 3.36 3.16 3.20 1.98 0.78 0.91MnO 0-12 0.]3 0.14 0.10 0.10 0.16MgO 0.96 0-80 1.68 0.97 0.29 0-30CaO 2.43 2.13 4.10 2.96 1.32 1.45Na~O 5-69 5.83 5-42 6.09 5-74 6.09K~O 6.51 6"57 5.94 6.62 7.28 7.12P~O~ 0-29 0.23 0-58 0.35 0.06 0'05H20+ 0-32 0.31 0.41 0.32 0.82 0-58I-I ~O'- 0"09 0.10 0-29 0.20 0-33 0.11Total 100'13 10 0.0 4 99'91 100.1] 100'43" 99'91* Includes 0-07 C1; less 0 equivalent of C1 = 0"05.

Or 38.36 38'92 35'03 38'92 43'37 42'26Ab 4C-87 42- 97 33. 54 36- 68 43- 49 44.01An 4.17 1.95 7.51 4.17 5.56 3.34Ne 3"98 3'41 6'53 7"95 2"84 3'98Di 5-35 6.53 7.60 6.80 1.05 2"33Ol 2'54 1'51 1"53 - - - - - -mt 2.09 2.55 3'02 2.09 1-39 2'32il 1.67 1.52 3.19 1.98 0.91 0-91ap 0'67 0"34 1'34 1'01 - - - -hm . . . . 0'48 - -Rest 0'41 0'41 0"70 0"52 1.21 0'74Total 100.11 100.11 99"99 100.12 100.30 99'87

Analyst: I. S. E. Carmichael.Solid solut ion in tern ary feldsp ars

The extent of sol id solut ion among anorthi te, alb i te, and potassiumfeldspars depends upon the temper atu re o f crys ta ll iza t ion and upon thecomposit i on of the l iquid from which the feldspars crystal l ized. Byanalogy with the experi mental results on the syst em NaA1Si04-KA1SiO4-Si02 -H2 0 (Turtle and Bowen, 1958 ; Ha mi lt on a nd Ma cKenzie , 1965), i tis to be expected that t rachytes wil l crystal l ize at h igher temperaturestha n ei ther rhyoli tes or phonoli tes, where the l iquidus te mper atur es are

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T R A C H Y T E S A N D T H E I R F E L I) SI ?A R P H E N O C R Y S T S 1 1 3l o w e r e d b y s i l i c a a n d n e p h e l i n e r e s p e c t i v e l y , s o t h a t i n t r a c h y t e s , t h ep h e n o c r y s t s o f p l a g io c l a se s h o u l d c o n t a i n l a r g e r p r o p o r t i o n s o f p o t a s -s i u m , a n d t h e a l k a l i- f e ld s p a r p h e n o c r y s t s l a rg e r a m o u n t s o f c a l c i u mt h a n t h e c o r r e s p o n d i n g p h e n o e r y s t s i n r h y o l i t e s a n d p h o n o l i t e s .

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TWO FELDSPARS

N a A I S i 3 0 8 m WtPlo K A I S i 3 08F IG . l . C o e x i s t in g fe l d s p a r p h e n o e r y s t s o f o v e r s a t u r a t e d t r a o h y t e s ( s o li d c ir cl es )a n d u u d e r s a t u r a t e d t r a c h y t e s ( o p e n c ir cl es ) p l o t t e d i n t h e t e r n a r y f e l d s p a r d i a g r a m .T h e d a s h e d l in e r e p r e s e n t s t h e l i m i t o f te r na r y- s ol id s o l u t i o n i n n a t u r a l f e l d s p a r s( S m i t h a n d M a c K e n z i e , 19 5 8 ; T u t t l e a n d B o w e n , 1 95 8). m r e p r e s e n t s t h e m i n i m u mo n t h e a l k a li f e l d s p a r j oi n a t 5 0 0 k g / o m 2 w a ~ er v a p o u r p r e s s u r e ( T t t tt le a n d B o w e n ,1 95 8 ), T h e s a n i d i n e 1 A , w h i c h h a s n o t b e e n a n a l y s e d ( t a b le I V ) , i s a s s m n e d t o h a v ea s i m i l a r e a l c i m n c o n t e n t t o t h e s a n i d in e : s 2 A a n d 3 A .

T h e a n a l y s e d c o e x i s t in g fe l d s p a r p h e n o c r y s t s ( t a b l e I V ) o f t h e t r a -c h y t e s d e s c r i b e d h e r e h a v e b e e n p l o t t e d i n f i g . 1 , a n d 5 A - 5 B i s t h e o n l yp l a g i o c l a s e - s a n i d i n e p a i r w h e r e e a c h f e l d s p a r s h o w s g r e a t e r t e r n a r ys o l i d s o l u t i o n t h a n t h e c o e x i s t i n g f e l d s p a r s i n r h y o l i t e s ( C a r m i c h a e l ,1 9 63 a) . T h e o t h e r p la g i o c la s e p h e n o e r y s t s h a v e s im i la r c o n t e n t s o f

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11 4 I. S. E. CAI~MICHAE L ONp o t a s s i u m t o t h e p i a g i o c l a s e p h e n o e r y s t s o f t h e t w o - f e l d s p a r r h y o l i t e s,a n d t h i s c o r r e sp o n d e n c e s u g g e st s t h a t t h e t e m p e r a t u r e d if fe r en c e b e -t w e e n t h e l i q u i d u s te m p e r a t u r e s o f t w o - f e l d s p a r r h y o l i te s a n d o ft r a c h y t e s m a y n o t b e g e n e r a l ly l a r g e e n o u gh t o a f fe c t t h e p o t a s s i u mc o n t e n t o f a p l a g i o c l a se . T h e t r a c h y t e s g e n e r a l l y c o n t a i n m o r e n o r to n -r i v e a n o r t h i t e t h a n t h e r h y o l i t e s (f ig . 2 : C a r m i c h a e l , 1 9 63 a ) a n d t h i sw o u l d h a v e t h e e ff ec t of r a is in g th e i r l i q u i d u s t e m p e r a t u r e s ; t h i s m a ya c c o u n t f o r th e g e n e r a l l y d if f er e n t t r e n d o f t h e t r a c h y t e f e l d s p a r t ie -l in e s ( fig . 1) c o m p a r e d t o t h e t r e n d o f t h o s e o f r h y o l i t e s , w h e r e t h e p l a g i o -c l a se is t y p i c a l l y n m r e s o d ic .

/ C a A I 2 S i 2 0 8/II\ \

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N a A i S i 3 0 8 , o R~W t.~ K A I S i 3 0 8FIG. 2 . The norm at ive te rna ry fe ldspar o f the under- and oversa tu ra ted t rachy tes(open circles, suffix R) an d the ir groundmasses (ov ersa tura ted, fi lled circles ; under-satura ted, f i lled t r iangles) p lot te d in the t ern ary feldsp ar d iagram. The lowerdashe d line represents pa rt of a general ized equi librium pa th on the sol idus of analka l i feldspa r crystal l iz ing in 4R or 1 0R . The u pper dashed l ine represents th el im it of ter t ia ry sol id solut ion (f ig . 1). The solid curve is the project ion on to theter na ry feldspar p lane of the two-feldspar surface as defined by the groundmasses

of the tr ac hy tes 11%, 2R, 3R, 5R, an d 6R.T h e m o r e p o t a s s i c o f t h e s a n i d i n e p h e n o c r y s t s f r o m t r a c h y t e s (f ig . 1

n o s . 1 A , 2 A , 3A ) h a v e s i m i l a r a m o u n t s o f c a l c iu m t o t h e s a n i d i n e p h e n o -c r y s t s o f t h e t w o - f e l d s p a r r h y o l i t e s . O f t h e r e m a i n i n g a l k a l i f e l d s p a rp h e n o c r y s t s , 4 A a n d 1 0 A h a v e t h e l a r g e s t a n o r t h i t e c o n t e n t. I t h a sp r e v i o u s l y b e e n s u g g e s t e d t h a t t h e a n o r t h i t e c o n t e n t o f 1 0A is h i g h ert h a n t h e m o r e p o t a s s i e s a n i d in e s f o u n d i n r h y o l i t e s b e c a u s e t h e l o w e rc o n t e n t o f s il ic a i n t h e r o c k 1 0 R h a d r a i s e d th e l i q u id u s t e m p e r a t u r e( C a r m i c h a e l , 1 96 3 a ). T h i s s u g g e s t i o n i s n o w c o n s i d e r e d t o b e e r r o n e o u s ,a s w i l l b e s h o w n b e l o w w h e n c o n s i d e r i n g th e f e l d s p a r - l i q u i d r e l a t i o n s h i p si n 4 R , a n d t h e a n o r t h i t e c o n t e n t o f b o t h 4 A a n d 1 0 A ( fig . 1) is c o n s i d e r e dt o i l l u s t r a t e t h e p r o g r e s s i v e s o l u b i l i t y o f a n o r t h i t e i n a n a l k a l i f e l d s p a ra s i t b e c o m e s m o r e s o d ic b y c o n t i n u o u s r e a c t i o n w i t h t h e l iq u i d . A

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T R A C H Y T E S A N D T H E I R F E L D S P A R P H E NO C IC Y S TS 1 ] 5g e n e r a l i z e d p a t h o n t h e s o l id u s o f a n a l k a l i f e ld s I ) a r c r y s t a l l iz i n g t 'r o m at r a c h y t e s im i l a r to 4 R w o u l d t h e re f o re r u n f r o m 3 A t o w a r d s 1 0 A a n d4 A ( fig s. 1, 2 ), u n t i l s u c h t i m e a s t h e l i q u i d b e c o m e s v i r t u a l l y o f c o n s t a n tc o m p o s i t i o n w i t h r e s p e c t t o N a z O : K 2 0 r a t i o , w h e r e u p o n t h e s o l i d u sp a t h o f t h e a l k a l i f e l d s p a r w o u l d c h a n g e d i r e c t io n t o w a r d s t h e a l k a l i-f e l d s p a r j o i n ( f i g . 4 ) .

Th e t w o - f e l d s p a r s u r f a ce a n d i t s e x t en tT h e b o u n d a r y c u r v e w h i c h r e p r e s e n t s l iq u i d s in e q u i l ib r i u m w i t h a

p l a g i o c l a s e a n d a s a n i d i n e i n t h e s y s t e m C a A 1 2 S i 2 0 s - N a A 1 S i s O s -K A 1 S i 30 s b e c o m e s a s u r f a c e i n t h e s y s t e m C a A lz S i2 O s -N a A 1 S iO 4 -K A 1 S i 0 4 - S i 0 2 , a n d i t w i l l b e d e f i n e d b y u n d e r - o r o v e r - s a t u r a t e d s a l i el i q u id s in e q u i l i b r i u m w i t h p l a g i o e l a se a n d s a n i d i n e . I t h a s p r e v i o u s l yb e e n s h o w n t h a t t h e t w o - f e l d s p a r s u r f a c e in t h e s y s t e m C aA 12 Si~ O -N a A 1 S i 3 O s - K A 1 S i s O s - S i 0 2 e x t e n d s , i n p r o j e c t i o n , o v e r t h e p o t a s s i c s id eo f t h e f e l d s p a r f ie ld i n t h e s y s t e m N a A 1 S i a O s - K A 1 S i 3 O s - S iO 2. T h et r a c h y t i e l iq u i d s d e f in i ng t h is s u r f a ce h a v e b e e n p r o j e c t e d o n t o t h eC a A 1 2 S i 2O s - N a A 1 S i3 O s - K A 1 S i3 0 s p l a n e i n f ig . 2 ( o v e r s a t u r a t e d l iq u i d s ,s o l id c i r c le s ; u n d e r s a t u r a t e d l iq u i d s , f il le d t r ia n g l e s ) . T h e t w o - f e l d s p a rs u r f a c e i n t h e s y s t e m C a A 1 2 S i ~ O s -N a A 1 S i O 4 - K A 1 8 i 0 4- S iO 2 i s u n l i k e l yt o p r o j e c t o n t o t h e t e r n a r y f e l d s p a r p l a n e ( C aA 1 2 S i2 O s -N a A 1 S i3 0 s -K A 1 Si3 O s) a s a l in e , b u t r a t h e r a s a n a r e a , s o t h a t t h e v a r i a t i o n i n a n -o r t h i t e c o n t e n t o f t h e t r a c h y t i c l iq u i d s w h i c h d e f i n e t h i s s u r f a c e ( fig . 2)m a y b e re a l. A n a l y t i c a l e r ro r , h o w e v e r , w i ll a c c o u n t f o r s o m e o f t h ev a r i a t i o n i n t h e a n o r t h i t e c o n t e n t , f o r t h e c a l c u l a t io n o f t h e C I P Wa c c u m u l a t e s m o s t o f t h e n o r m a l e r ro r s o f c h e m i c a l a n a l y s is o n th e p r e s -e n c e o r a m o u n t o f n o r m a t i v e a n o r t h i t e . I n o r d e r t o s im p l i fy a l a t e rd i sc u s si o n , a n a p p r o x i m a t e c u r v e h a s b e e n d r a w n i n fig . 2 t h r o u g h t h eu n d e r - a n d o v e r s a t u r a t e d l iq u id s p r o j e c t e d o n to th e t e r n a r y f e l d s p a rp l a n e , a n d r e p r e s e n t s l i q u id s ( in t e r m s o f t h e t h r e e f e l d s p a r c o m p o n e n t s )i n e q u i l i b r iu m w i t h p la g i o c la s e a n d s a n i d in e . E v e n w i t h t h is a p p r o x i -m a t e c u r v e i t is c le a r t h a t f o r th e t h r e e i n i t i a l o v e r s a t u r a t e d c o m p o s i t i o n s1 R , 2 R , a n d 3 R , a p l a g i o c l a s e w o u l d p r e c i p i t a t e b e f o r e a n a l k a l i f e l d s p a r ,a n d t h e s a m e i s t r u e f o r t h e u n d e r s a t u r a t e d r o c k 6 R .

T h e tw o o v e r s a t u r a t e d t r a c h y t e s 4 R a n d 1 0 R (fig . 2 ) h a v e t h e l e a s ta m o u n t o f n o r m a t i v e a n o r t h i t e , a n d y e t t h e y h a v e s a n id i ne p h e n o c r y s t s4 A a n d 1 0 A w i t h t h e h i g h e s t a n o r t h i t e c o n t e n t (fig. 1) . T o u n d e r s t a n dt h i s a p p a r e n t p a r a d o x , i t is n e c e s s a r y t o i l lu s t r a te a c o u r se o f e q u il i-b r i u m c r y s t a l l i z a t i o n f o r a l i q u i d s i m i l a r t o 4 R .

T h e c o m p o s i t i o n 4 R ( in p r o je c t io n ) f a ll s n e a r t h e i n t e rs e c t i o n o f t h e

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1 1 6 I. S . E . CAR,M].CHAEL ONSiO 2

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N B A I S i 3 0 8 4 * Io A ,A 2A3A K A I S i 3 0 8W t . ~F :~ o. 3 . T h e n o rm a t iv e s a l ie c o n s t i tu e n t s ( le ss a n o r th i t e ) o f th e s a l ie p h a s e s o f tw o -fe ld s p a r o v e r s a tu r a t e d t r a e h y te s p lo t t e d in th e s y s t e m N a A lS ia O s-K A 1S ia O s S i0 2 .G ro u n d ma s s c o mp o s i t io n s a re r e p re s e n te d b y o p e n c i r c l e s ( s u f f ix G ) a n d a re jo in e dt o t h e i r r e s p e c t i v e s a n i d i n e a n d p l a g io c l as e p h e n o e r y s t s ( r e p r e s e n t e d a s p u r e a l b i te ) .T h e ro c k s a re r e p re s e n te d b y s o l id c ir c le s w i th su ff ix 1 ~. T h e c o m p o s i t io n 4 R (a n d1 0 g , n o t p l o t t e d ) f a i ls o u t s i d e t h e t r i a n g l e w h o s e a p i c es r e p r e s e n t t h e g r o u n d m a s s4 G , t h e s a n i d i n e 4 A a n d t h e p l a g i o e l a s e ( p u r e a l b i t e ) , b e c a u s e t h e s a n i d i n e ( 4 A )c o n t a i n s s i g n if i c a n t a m o u n t s o f t il e a n o r t h i t e c o m p o n e n t ( t a b l e I V ) a n d i t h a s b e e np l o t t e d o n l y i n te r m s o f i t s O r c o n t e n t . T h e b o u n d a r y c u r v e a n d m i n i m u m ( re -p r e s e n t e d b y + ) a t 5 00 k g / e m 2 w a t e r v a p o u r p r e s su r e a r e t a k e n f ro m T u r t l e a n dB o w e n ( 19 58 ), t o g e t h e r w i t h t h e t h e r m a l v a l le y , r e p r e s e n t e d a s a d a s h e d li n e .t h e r m a l v a l l e y 1 w i t h t h e M k a l i - f e ld s p a r j o i n ( fig . 3 ) a n d T u r t l e a n dB o w e n ( 19 5 8 ) h a v e d i s c u s s e d a t l e n g t h t h e c r y s t a l l i z a t i o n o f c a l c iu m - f r e es y n t h e t i c l i q u id s o f o t h e r w i s e s i m i l a r sa li e c o m p o s i t i o n t o 4 R . T h e y h a v es h o w n t h a t t h e f i r s t f e l d s p a r to p r e c i p i t a t e f r o m t h e s y n t h e t i c e q u i v a l e n to f 4 R w o u l d b e c o n s i d e r a b l y m o r e p o t a s s i e t h a n t h e i n i t i a l c o m p o s i t i o n

1 I t h a s b e e n a s s u m e d t h a t t h e u n i q u e f r a e t i o n a t i o n c u rv e a n d t h e ' t h e r m a lv a l l e y ' ( T u r t l e a n d B o w e n , 1 95 8) a r e c o i n c i d e n t i n b o t h t h e o v er - a n d u n d e r -s a t u r a t e d p a r t s o f p e ~ r o g e n y ' s r e s i d u a l sy s t e m ( N a A 1 S iO ~ - K A 1 S i Q - S iO 2 )

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TRACHYTES AND THEIR FELDSPAR PI tENOCRYSTS 117o f t h e l iq u id, s o t h a t i t is r ea s o n a b le to s u p p o se t h a t t h e c o mp o s i ti o n o fthe f i rs t c rys ta l s o f san id ine to p rec ip i ta te f rom 4R wou ld al so be m orepo tass ic, and cou ld be s imi la r to th e san id ines 1A -3A ( f ig . 3 ). I t i sthe re fo re poss ib le to deduce a genera l ized pa th on the so l idus o f ana lka l i f e ldspar p rec ip i ta t ing under equ i l ib r ium cond i t ions f rom a l iqu idsim ilar to 4R (f ig . 4).

As the in i t i a l compos i t ion o f 4R p lo t s be tween the a lka l i - fe ldsparsol idus pat h an d th e a lkal i - fe ldspa r jo in ( f ig . 4), i t i s poss ib le to exp ressth e s c o m p o n e n t s o f t h i s r o c k s o le ly i n t e rms o f a c a lc iu m-bear ing a lka li - fe ldspar , so th a t the p lag ioe lase phen ocry s t s 4B ( f ig . 4 )wi l l be reso rbed a t some s tage dur ing equ i l ib r ium c rys ta l l i za t ion(Tur t le and Bowen , 1958) .

F o l lo win g Tu r t l e a n d Bo w e n (19 58 ), i t m a y b e c o n c lu d e d th a t a p o t a s -s ic a lka li - fe ldspar was the f i r s t f e ldspar to p rec ip i ta te f rom 4R , and wi thcon t inued equ i l ib r ium coo l ing , the san id ine , moving a long the so l iduspa th in fig . 4 , becom es m ore sod ie , and a t the same t im e i s p rogress ive lye n r i c h e d in a n o r th i t e .

A t s o me s ta g e t h e l iq u id i n t e rs e c t s t h e tw o - f e ld s p a r s u r fa c e so t h a t ap lag ioclase beg ins to p rec ip i ta te toge th e r w i th the san id ine . Th e ap-p a re n t p a r a d o x i s th e r e s u l t o f t h e l o w c o n te n t o f a n o r th i t e i n t h e i n i t i a ll iqu id d e lay ing the p rec ip i ta t ion o f the p lag ioclase so a llowing the san i-d in e t o b e c o me in c r e a s in g ly s o d i e a n d a t t h e s a me t ime e n r i c h e d ina n o r th i t e .

Th e p e t ro g ra p h ic e v id e n c e o b t a in e d f ro m a s tu d y o f 4 R in d i c a t e s t h a tthe p lag ioc lase enc losed in some o f the san id ine phe noc rys t s has be enp a r t i a l l y r e s o rb e d . Th e re fo r e w i th t h e c o n t in u e d p r e c ip i t a t i o n o f s a n i-d ine and i t s reac t ion wi th the l iqu id to become inc reas ing ly sod ic , theaccom pany ing p lag ioclase wil l beg in to be reso rbed . As the t i e - l inebe tw een p lag ioc lase 4B a nd the l iqu id 4G a l so co n ta ins th e san id ine 4A(f ig . 4) , i t i s poss ib le to express the composi t ion of 4R, which a lso l ies(w i th in e x p e r im e n ta l e r ro r) o n t h e s o l i d - l iq u id t i e -l in e 4 B -4 A -4 G ,e i the r in te rms o f p lag ioc lase 4B and l iqu id 4G, o r o f san id ine 4A andl iqu id 4G. H ow ever , i t has been shown befo re th a t fo r sa lic l iqu ids whichp lo t on the po tass ie s ide o f the t he r m al va l ley , i t i s the p lag ioc lase ra the rthan the san id ine which i s re so rbed (Carmichae l 1963a) .

P e rh a p s i t m a y b e s u g g e s te d t h a t wh e n th e s o li d us p a th o f t h ea lka l i - fe ldspar in te r sec t s the p lag ioc lase - l iqu id th ree -phase boundary4B -4G , o r , in o the r w ords , wh en the th ree -p hase t r iang le o f l iqu id ,san id ine and p lag ioc lase becomes a s t ra igh t l ine wi th l iqu id remain ing ,the reso rp t ion o f one o f the so l ids , namely p lag ioc lase , shou ld be

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1 1 8 I . S . E . C A R M I C H A E L O Nc o m p l e t e u n d e r e q u i l i b r i u m c o n d i t i o n s , a n d t h e l i q u i d c a n t h e n l e a v et h e t w o - f e l d s p a r s u r f a c e . I t i s d i f f i c u l t t o a s s e s s w h e t h e r o r n o t , o r t ow h a t e x t e n t , t h e p a r t i a l r e s o r p t i o n o f p l a g i o e l a s e h a s c o n t r i b u t e d t o t h ec o m p o s i t i o n o f t h e s a n i d i n e 4 A .

W i t h c o n t i n u e d e q u i l i b r iu m c o d i n g , t h e p l a g io c l a se w i l l b e c o m p l e t e l yr e s or b e d , a n d t h e i n c o r p o r a t i o n o f it s c o m p o n e n t s i n t o t h e l iq u i d m a y

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N a A I S i 3 0 8 ,G Wt?lo K A I S i 3 0Fro . 4 :. The pl~gioclase (4B ) and the sanidine (4A ) pheno crysts of the t rac hy te4R are p lo t ted in the te rn ary fe ldspar d iagram together wi th the norm at ive te rna ryfeldspar of 4R and the groundmass 4G. The dashed curve represents the l im it ofter na ry solid solution (fig. 1); the dash-clot curve S- 4A -4 R represents an equili-b r ium pa th on the sol idus o f an a lka l i Mdspa r c rys ta l l iz ing rom a l iqu id rep resen tedby 4I~; the da sh- do t curve P -4B represents a n equil ibrimn solidus path of theplagioclase phenocrysl,s. The alka li-feld spa r minimum at 500 kg/em 2 w ater va po ur(double arrows) is taken from Tu tt le a nd Bow en (1958).k e e p t h e s a n i d i n e m o r e o r le s s c o n s t a n t i n c o m p o s i t i o n , f o r th e p r e c i p i t a -l i o n o f s a n i d i n e 4 A ( fig . 3 ) t e n d s t o e n r i c h t h e l i q u i d i n p o t a s s i m n , a n di m p o v e r i s h i t i n a n o r t h i t e , w h e r e a s t h e r e s o r p t i o n o f p l a g i o c la s e t e n d s t oe n r i c h i t i n s o d i m n a n d a n o r t h i t e , s o t h a t f o r a t i m e t h e s e tw o e f fe c tsm a y b a l a n c e . W i t h c o n t i n u e d c o o l i ng a n d p r e c i p i t a t i o n o f sa n id i n e, t h el i q u i d m o v e s t o w a r d s h i g h e r s i l i c a c o n t e n t s , u n t i l q u a r t z o r t r i d y m i t e p r e -c i p i t a t e . W i t h t h e p r e c i p i t a t i o n o f a si l i c a m i n e r a l , t h e s a n i d i n e , h a v i n gb e c o m e s t e a d i l y i m p o v e r i s h e d in a n o r t h i t e b y c o n t i n u o u s r e a c t i o n w i t ht h e l i q u id , w i l l n o w r e v e r s e i t s i n i t i a l t r e n d o f so d ic e n r i c h m e n t , a n db e c o m e i n c r e a s i n g l y p o t a s s i c ( fig . 4 ). T h e c o m p o s i t i o n 4 R w i l l b e c o m e a l ls o l i d w h e n t h e t i e - l i n e ( in t h e s y s t e m C a A 1 2 S i~ O s - N a A 1 S ia O s - K A 1 S i3 0 -S i O ~ ) f r o m t h e s a n i d i n e , p l o t t e d o n t h e t e r n a r y f e l d s p a r p l a n e , t o t h es i li c a a p e x i n t e r s e c t s t h e i n i t i a l s a li c c o m p o s i t i o n p l o t t e d i n te r m s o ft h e s e f o u r c o m p o n e n t s .

T h e i n i t i a l l i q u i d s 2 R a n d 3 R ( fi g. 3 ) a l s o l ie n e a r t h e t h e r m a l v a l l e y ,a n d a n e q u i l i b r i u m c o u r s e o f c r y s t a l l i z a t i o n s i m i l a r t o 4 R i s t o b e

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TRACHYTES AND THEIR FELDSPAR PHENOCRYSTS 119e x p e c t e d fo r b o th o f th e m . Ho we v e r , a s b o th o f th e s e l iq u id s c o n t a inm o re a n o r th i t e t h a n 4 R (fig . 2 ), a n d p lo t a b o v e t h e tw o - f e ld s p a r b o u n -da ry su r face ( in p ro jec t ion) , p lag ioc lase is the f i r s t f e ldspar to p rec ip i ta te .M o reo v e r, as t h e a n o r th i t e c o n t e n t o f t h e i n it i al l iq u id s is g r e a t e r t h a ntha t he ld in the san id ine phenocrys t s , p lag ioc lase wi l l con t inue to bep re s e n t t h ro u g h o u t t h e c ry s t a l li z a t io n in t e rv a l .

Th e i n i t ia l p r e c ip i t a t i o n o f p l ag io c la s e f ro m 2 R a n d 3 R e n r ic h e s t h el iq u ids i n p o t a s s iu m a n d imp o v e r i s h e s t h e l iq u ids i n a n o r th i t e u n t i l t h e yimpinge on the two- fe ldspar su r face , where po tass ie san id ines beg in toprec ip i ta te ( fig. 3 ; 2A, 3A). Th erea f te r the p rec ip i ta t ion o f p lag ioc lasee x e r t s a c o n t in u o u s t e n d e n c y to e n r i c h t h e l i q u id s i n p o t a s s iu m a n d s oprev en ts the san id ine phe noc rys t s becom ing inc reas ing ly sod ic . I t i ssugges ted , the re fo re , th a t in liqu ids o f s imi la r comp os i t ion to 2R an d 3R ,th e c o mp o s i t i o n o f t h e e a r ly s a n id in e w ill n o t c h a n g e m a rk e d ly i n t e rm so f i t s Na 2 0 :K~ O r a t i o d u r in g t h e c ry s t a l li z a t io n i n t e rv a l , b u t t h e c o n -t i n u e d r e a c t i o n o f t h i s f e ld s p a r w i th t h e l i q u id t o m a in t a in e q u i l i b riu mwill p rogress ive ly dec rease i t s ano r th i te con te n t . Th e p lag ioelase wi l ls im i la r ly d e c r e as e i n a n o r th i t e , b u t t h e N a 2 0 :K 2 0 r a t i o m a y a l so o n lychange wi th in na r row l imi t s un t i l the l iqu id has been used up (Car -m ichael , 1963a).

These tw o eq u i l ib r ium courses o f c rys ta l l i za t ion i l lus t ra te th e genera lc o n c lu s io n t h a t t h e c ry s t a l l i z a t i o n o f n o rma t iv e a n o r th i t e -b e a r in gp o ta ss i c t r a c h y te s , wh ic h i n p ro j e c t i o n p lo t o n t h e p o t a s s ic s i d e o f t h ethe rmal va l ley in f ig . 3 , canno t be desc r ibed so le ly in re la t ion to thesys tem NaA1Si3Os-KA1SiaOs-SiO 2.

Th e two u n d e r s a tu r a t e d t r a c h y t i e l i q u id s 5 G a n d 6 G ( t a b l e I I I ) ,which a re in equ i l ib r ium w i th p lag ioc lase and san id ine phen oerys t s ,h a v e b e e n p lo t t e d i n f i g . 5 , a n d i n d i c a t e t h a t t h e two - f e ld s p a r s u r f a c ee x t e n d s i n p ro j e c t i o n t o wa rd s t h e n e p h e l in e - f e ld s p a r b o u n d a ry .

Th e l a t e r a l e x t e n t o f t h i s s u r f a c e t o wa rd s t h e t h e rm a l v a l l e y is u n -k n o w n , b e c a u s e i t h a s b e e n s h o w n t h a t t h e t h e r m a l v a l l e y a n d t h emin im u m o n th e n e p h e l in e - f e ld s p a r b o u n d a ry c u rv e ( f i g . 5 ) a r e c o n -s ide rab ly d i sp laced ( in p ro jec t ion) towards more po tass ie compos i t ionsfo r u n d e r s a tu r a t e d l iq u id s w h ich p lo t i n t h e f e ld s p a r f ie ld a n d c o n ta innormat ive anor th i te (Carmichae l , 1964) .

Th e two n n d e r s a tu r a t e d i n i t i a l l i q u id s 7 R a n d 8 R h a v e a s im i l a rc o mp o s i t i o n (t a b le I I I ) , a l t h o u g h 7 R d o e s c o n t a in a l i t t le m o re n o rma -t i v e a n o r th i t e ( fi g. 2 ). H o w e v e r t h e s a n id in i t e 8 R , w h ic h h a s p r e s u m a b lyc o o led s lo wly a n d th e r e fo r e a p p ro a c h e d e q u i l i b r iu m c o n d i t io n s , h a s o n lyv e ry r a r e r e m n a n t s o f p a r t i a l l y r e s o rb e d p l a g io c la s e e n c lo s e d b y s a n id in e,

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120 I. S. E. CARMICHAEL ONso th a t the san id ine 8A ( tab le IV) co r respon ds c lose ly to the compos i -t ion o f the no rm at ive fe ldspar in the in i t i a l l iqu id (8R, tab le I I I ) . Th isc o r r e s p o n d e n c e i n d i c at e s t h a t a s a n u n d e r s a tu r a t e d l i q u id c ry s t a ll iz e su n d e r e q u i l i b riu m c o n d i t io n s a n d m o v e s to w a rd s t h e n e p h e l in e - f e ld s p a rbo un da ry ( f ig . 5 ), the p lag ioc lase wil l, d epend ing up on the l iqu id ' sin i t ia l com pos i t ion , be com ple te ly reso rbed .

N aA I S i O W t . ~

OF

K A I S i OFro. 5. The normati ve salie constituents (less anorthite) of the groundmasses of theunders aturat ed tr achyte s plot ted in the sys tem NaA]SiO4-KA1SiO4-SiO 2. Thenepheline-feldspar field bound ary and mini mum at 1000 kg/cm ~ are take n fromHamilt on and MacKenzie (1964) togethe r with the pos ition of the the rma l valley(dashed line). The alkali feldspar mini mmn and the minimum on the bo unda rycurve ~re represented by double arrows.

Although t i l e a ssemblage nephe l ine p lus p lag ioe lase p lus san id ine inequ i l ib r ium w i th liqu id is appa ren t ly ra re in na tu re , th i s does no t neces -s a r i l y i n d i c a t e t h a t t h e two - f e ld s p a r s u r f a c e d o e s n o t e x t e n d to t h en e p h e l in e - f e ld s p a r b o u n d a ry ( i n p ro j e c t i o n i n f i g . 5 ) , b u t r a th e r t h a tt h e c o mp o s i t i o n o f m o s t u n d e r s a tu r a t e d s a li e li q uid s (wh ich p lo t i n t h efe ldspar f ie ld in f ig . 5) requi tes that i f p lagioelase is precip i ta ted , i t i ssubs equ en t ly reso rbed . Th e incorp ora t ion o f the p lagioclase com po-n e n t s a s a r e s u l t o f r e s o rp t i o n ma y a f f e c t t h e c o mp o s i ti o n s o f b o th t h ea lka l i f e ldspar and the nephe l ine , bu t a ssuming tha t nephe l ine con ta insa p p ro x im ' ~ t e ly t h e s a me a m o u n t o f t h e a n o r th i t e m o lec u l e a s a n a lk a life ldspar , then the c r i t e r ion , tha t i f the re i s more anor th i te in the in i t i a ll iquid than in the san~dine phcnoerysts , then a p lagioclase wil l exis tth ro ug ho u t th e c ry s ta l l i za t ion in te rva l , i s st i ll va lid .

The p rec ip i ta t ion o f p lag ice lase and i t s l a te r re so rp t ion dur ing theequ i l ib r ium c rys ta l l i za t ion o f m an y oversa tu ra te d po tass ie sal ie l iqu idsind ica tes tha t p lag ioe lase wi l l behave s imi la r ly in the undersa tu ra ted

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T R A C H Y T E S A N D T H E I R F E L D S P A R P H E N O C R Y S T S 1 2 1p o t a s s i c s a l i c l i q u i d s , s o t h a t d e p e n d i n g u p o n t h e c o m p o s i t i o n o f t h ein i t i a l l iqu id , the two- fe ldspar su r face i s e i the r a co -p rec ip i ta t ion o r ar e s o rp t io n s u rf a c e, o r c h a n g e s f ro m o n e t o t h e o th e r d u r in g c ry s t a ll i z a -t io n . Th e two f e ld s p a r su r f a ce is t a k e n h e r e t o e x t e n d in p ro j e c t i o n t othe nephel ine- fe ldspar bo un da ry curve ( f ig . 5 ).

T]w salic tetrahedron." the systemCaAl~Si2Os-NaA1SiOa-KA1SiOaSiO 2

W i th t h e d a t a p ro v id e d b y t h e c o mp o s i t io n s o f t h e s a lic p h a s e s o fv o l c a n ic r o c k s wh o s e c o mp o s i t i o n c a n b e e x p re s s e d a lmo s t c o m p le t e ly i nt e rm s o f t h e a b o v e fo u r c o m p o n e n t s , i t i s p o s sib le t o r e p r e s e n t s c h e ma -t i c a l l y s o me o f t h e r e l a t i o n s h ip s i n t h i s f o u r - c o mp o n e n t s y s t e m, a tu n k n o w n b u t m o d e r a t e w a t e r v a p o u r p r es su re s.

In f ig . 6 , a hy po the t i ca l rep r esen ta t ion o f the sal ic t e t r ah ed ron i sg iven , and i s s imi lar to tha t desc r ibed by Sobo lev (1959). Th e base o fthe te t ra he dro n is t he sys tem NaA1SiOa-KA1SiOa-S iO e and w i th in thet e t r a h e d ro n th e s u r f a c e ABCDE rep resen ts the two- fe ldspar su r face .Th i s s u rf a c e c a n n o t i n t e r s e c t t h e b a s e o f t h e t e t r a h e d ro n , h o w e v e rc lose ly i t m ay app roach i t , a s the equ i l ib r ia o f the a lka l i f e ldsparse i the r a lone o r wi th nephe l ine o r s i l i ca a re charac te r ized by min imara th e r t h a n b y e u t e c ti c s , wh ic h wo u ld b e n e c e s s i t a te d i f AE i n t e r s e c t e dth e b a s e o f t h e t e t r a h e d ro n . Th e two - f e ld s p a r s u r fa c e m u s t f a d e o u ta long AE a s i t a p p ro a c h e s th e m in ima (M , M ' a n d M " ) i n th e b a s e o f t h et e t r a h e d ro n . A t h ig h pr e ss u res o f wa te r v a p o u r t h e m in ima wi ll o fcourse be rep laced by eu tec t ic re la t ions (Tur t le and Bowen , 1958) .

Th e c u rv e ED r e p re s e n t s t h e i n t e r s e c ti o n o f t h e two - f e ld s p a r su r f a c ewi th t h e b o u n d a ry s u r fa c e s e p a ra ti n g t h e s ilic a v o lu m e f ro m th e p l ag io-c lase vo lum e, and wi ll be de f ined by l iqu ids in equ i l ib r ium w i th qua r tz ,san id ine , an d p lag ioc lase , nam ely the po rph yr i t i c rhyo l i te s (Carmichae l ,1 9 6 3 @

T h e c u r v e BC rep resen ts l iqu ids in equ i l ib r ium wi th p lag ioc lase ,san id ine , and leuc ite . F r om B a four -phase cu rve (now shown) ex te nd su p w a rd s i n to t h e t e t r a h e d ro n a n d i s d e f in e d b y l iq u ids i n e q u i l i b r iu mw ith p lag ioc lase , nephe l ine , and leuc i te . Th is four -phase cu rve is a t th eapex o f two in te r sec t ing su r faces , the nephe l ine -p lag ioc lase su r face an dthe leuc i te -p lag ioc lase su r face . Th e po in t B i s an inv ar ia n t po in t ( ina condensed sys tem) where l iqu id i s in equ i l ib r imn wi th nephe l ine ,leuc i te , san id ine , and p lag ioc lase , a ll o f f ixed comp os i t ion . A t ra ch y t ef rom R om e, I ta ly , in th e Im per ia l Co l lege co llec t ions has leuc i te , san i-d ine , and p lag ioc lase phenocrys t s wi th nephe l ine su r round ing some o f

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12 2 I . S . E. CARM ICHAEL ONC a A l 2 S i 2 0 8

/ \ \ \ k/ % / \\ \

/ \/ /

NephelinesLeucite

N ~ A I S i O K A ~ SiOFi e . 6 . A genera l ized schem at ic represc:~ta tion of the sys tem CaAI2Si20s -NaAISiO4-KA1SiO4-SiO ~ as deduced f rom porph yri t ic sa l ic volcanic rocks . M,M',and M" rep re sen t the m in ima on the qua r t z - fe ld spa r boun da ry cu rve , the a lka l if e ldspa r jo in and the nephe l ine - fe ldspa r bounda ry cu rve re spec t ive ly . ABCDErep re sen t s the two - fe ldspa r su r face above the base o f the t e t rahe dron ( see t ex t ) .Th e re la t ionships for the sys tem KA1Si~O~-OaA12Si2Os-Si02h a v e b e e n t a k e n f r o mSchairer and Bow en (1947); for the sys tem CaA12Si2Os-NaA1Si04-SiO ~ f romScha i re r (in Roedde r , 1959), and fo r the base o f the t e t rah edron f rom Tur t l e andBowen (1958) and Hami l ton and MacKenz ie (1964) .t h e s a n id i n e , w h i c h i n d i c a t e s t h a t t h e l i q u id m a y h a v e a c o m p o s i t i o nc lo s e t o B , o n t h e c u r v e BC.

T h e c u r v e AB i s t h e t r a c e o f t h e i n te r se c ti o n1 o f t h e t w o - f e l d s p a rs u r f a c e ABCDE w i t h th e b o u n d a r y s u r fa c e s e p a r a t i n g t h e n e p h e l in ev o l u m e f r o m t h e p l a gi o c la s e v o l u m e . A B w i ll b e d e f in e d b y l iq u i d s ine q u i l i b r i u m w i t h p l a g io c l a s% s a n i d i n e a n d n e p h e l i n e .

N a t u r a l s a li e l i q u i d s w h i c h i n p r o j e c t i o n p l o t i n t h e f e l d s p a r f i el d o nt h e s o di c s id e of t h e t h r e e m i n i m a M , M ' , a n d M " i n t h e b a s e o f t h et e t r a h e d r o n a r e r h y o l i te s , t r a c h y t e s , a n d p h o n o l i te s w i t h p h e n o c r y s t s

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TRACHYTES AND TH EIR FELDSPAR PffENOCt/~u 12 3of the p lag ioc lase -ano r thoc lase sol id so lu t ion ser ie s. A t no t im e in thec rys ta l l i za t ion in te rva l o f these l iqu ids wi ll the p lag ioc lase -anor thoc lasefe ldspar be jo ined by a potass ie sanidine (Carmichael , 1963a) .

Th e a d d i t i o n o f a n o r th i t e t o c o mp o s i t i o n s i n t h e b a s e o f t h e t e t r a -hed ron (NaA1Si0a-KA1Si0a-S i02) ra ises the l iqu idus te m pe ra tu re(unpub l i shed work) so th a t l iqu ids , co r respon d ing to the r es t r ic ted rang eof com pos i t ion o f na tu ra l sa lic rocks , wil l a l l t en d to m ove tow ards t heb a s e o f t h e t e t r a h e d ro n . Th e g r e a t e r th e d e g re e o f f r a c t i o n a t io n , t h eg re a t e r t h e t e n d e n c y fo r t h e l i q u id t o r e a c h t h e b a s e o f t h e t e t r a h e d ro n .

ConclusionsFrac t iona l c rys ta l l i za t ion o f l iqu ids in the sa l ic t e t rahedron dep le testh e l i q u id s i n t h e a n o r th i t e c o mp o n e n t , s o t h a t d e r iv a t i v e l i q u id s ma yb e c o m e c o mp le t e ly f r e e o f t h e a n o r th i t e m o lec u le . An e x a m p le o f at r e n d in t h i s d i r e c t i o n is g iv e n b y t h e v o l c a n ic r o c k s o f Go u g h I s l a n d ,wh e re t h e p l a g io c la s e -s a n id in e t r a c h y te s ( 5 R , t a b l e I I I ) a r e c o n s id e r e dto b e h ig h e r u p a l i q u id li n e o f d e s c e n t t h a n t h e a n o r th o e l a s e - a e g i r i n e -aug i te t rachy tes (Le Mai t re , 1962) .

Th e c rys ta l l i za t ion o f p lag ioc lase and i t s l a te r com ple te o r pa r t ia lr e s o rp t io n in m a n y s alic l i q u id s m a y b e a c c e n tu a t e d , o r e v e n c a us e d , b yth e p r e f e r e n t i a l i n c o rp o ra t io n o f c a l c iu m (v is & v is a n o r th i t e ) f r o m th el iqu id ; th i s has been ca l led the ' p lag ioc lase e ffec t ' (Bowen , 19 t5 ;Carm ichae l and M acKenz ie , 1963). Ind eed the two- fe ldspa r su r facem a y o n ly b e c o m e a r e s o rp t io n s u r f a c e wh e re c a l c iu m (o th e r t h a n a s ac o n s t i t u e n t o f t h e a n o r th i t e mo le c u l e ) i s p r e s e n t i n t h e l i q u id , b u twh e th e r o r n o t th i s is so c a n n o t b e s h o wn wi th t h e d a t a o f n a tu r a l s alicl iqu ids . Th e f rac t iona l c rys ta l l i za t ion o f sa lic liqu ids where ca lc ium hasbeen p re fe ren t ia l ly inco rpora ted in to the p lag ioc lase will p roduce und er -s a tu r a t i o n o f A l s Oa w i th respec t to to t a l a lka li s in t he de r ived liqu ids ,s o t h a t a lka l i p y ro x e n e s a n d /o r a mp h ib o le s m a y b e c o me p ro m in e n t inthe fe r rom agnes ian assemblages o f these f rae t ion a ted p roc luc ts (e f. LeMaitre, 1962).

I t has p rev ious ly been sugges ted (Carmichae l, 1983a) tha t f rac t ion a lc ry s t a l l i z a t i o n o f ' n o rma l ' b a s a l t i c ma g ma wo u ld o n ly p ro d u c e o n e -fe ldspar sal ie l iqu ids, the fe ldspar be ing a m em ber o f the p lag ioc lase -ano rtho clas e solid solut ion ser ies . I f tw o-fe ld spa r sa lie, par t i cu lar lyo v e r s a tu r a t e d , l i q u id s a r e a c c o u n te d d e r iv a t i v e s b y f r a c t i o n a l c ry s t a l -l i z a ti o n o f b a s a l ti c ma g ma , t h e n t h e p a r e n t a l b a s a l t m u s t i t s d f b e p o t a s -s ic . Th e p rob lem of the o r ig in o f the tw o- fe ldspar po tass ic salic l iqu idsth e r e fo r e b ec o me s a p ro b l e m o f t h e o r ig in o f t h e p o t as s ic n a tu r e o f th e

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1 2 4 I . s . E . C A ~M ~ CU A EL O Np a r e n t a l b a s a l t , w h i c h in t h e c o n t i n e n t a l a re a s , m a y b e c o n v e n i e n t l ya s c r i b e d t o s ia li c c o n t a m i n a t i o n .

I n t h e o c e a n i c a r e a s w h e r e s i a li e c o n t a m i n a t i o n is l e s s e a s i ly a c c e p t e d ,C h a y e s ( 1 9 6 3 ) f o u n d a r e l a t i v e p a u c i t y o f i n t e r m e d i a t e r o c k s i n t h eb a s a l t - t r a e h y t e s e ri e s, a n d c o n c l u d e d t h a t ' t h e r e l a t i v e a b u n d a n c e o fs a li c o c e a n i c l a v a s d o e s n o t t e l l u s w h e t h e r c r y s t a l f r a c t i o n a t i o n o fb a s a l t i c m a g m a i s n e c e s s a r y f o r t h e i r g e n e r a t i o n b u t s u g g e s t s v e r ys t r o n g l y t h a t i t is i n s u f f i c i e n t ' .

Acknowledgements. D r . H . C a r s te n s , D r . R . W . L e M a i t r e , a n d t h e T r u s t e e s o f t h eB r i t i s h M u s e u m ( N a t u r a l H i s t o r y ) k i n d l y p r o v id e d m a n y o f t h e t r a c h y t e s p ec i-m e n s . T h e a u t h o r i s a g a i n i n d e b t e d t o D r . W . S . M a c K e n z i e f o r h i s c o n t i n u e da d v i c e o n f e l d s p a r p r o b l e m s a n d f o r h i s c r i t i c i s m d u r i n g t h e p r e p a r a t i o n o f t h em a n u s c r i p t .

P e t r o g r a p h i c a T p e n d i xThe oversaturated trachytes. P h e n o e r y s t s o f w e a k l y z o n e d p l a g io c la s c ( 1 B ) f o r mi n t e r l o c k in g c r y s t a l c lu s t e r s i n t h e p i t c h s t o n e 1 R . P a l e g r e e n a u g i te , b i o t i t e , b r o w na m p h i b o l e ( b a r k e v i k i t e ) , a n d r a r e s a n i d i n e ( 1 A ) o c c u r a s s m a l l e r w e l l - s c a t t e r e dp h e n o c r y s t s a n d a r e a l l e n c lo s e d b y a b r o w n v e s i c u l a r g l as s (1 G ) . P h e n o e r y s t s i nm i n o r a m o u n t s a r e s p h e n e , a p a t i t e , z i,'e on , a n d i r o n - o r e, t h e l a s t c o m m o n l y b e i n g

e n c lo s e d b y t h e a u g i t e p h e n o e r y s t s . N e i t h e r t h e b i o t i t e n o r t h e a m p h i b o l e p h e n o -e r y s t s s h o w a n y e v i d e n c e o f r e s o r p t i o n b y t h e g l a ss , w h i c h h a s c o n g e al e d w i t hs c a t t e r e d m i n u t e f e l d s p a r l a t h s t o g e t h e r w i t h p y r o x e n e a n d i r o n - o r e g r a n u l e s .2 g i s t h e c e l e b r a t e d t r a c h y t e t ~ 'o m D r a c h e n fe l s, G e r m a n y , w h i c h h a s l ~ rg ei d i o m o r p h i e s a n i d i n e p h e n o c r y s t s ( 2A ) u p t o 2 in . i n l e n g t h t h a t m a y e n cl o sep la g io cla s e , b io t i t e , a n d s p h e r ic . T h e a b u n d a n t p l a g io e la se p h e n o e ry s t s (2 B ) c o m-m o n l y c l u s te r , a n d t o g e t h e r w i t h a u g i t e, a n d b i o t i t e i n a ll s t a g e s o f r e s o r p t i o n a n dr e p l a c e m e n t b y a g g r e g a t e s o f i r o n -o r e , a r e e n c lo s e d i n a g r o u n d m a s s ( 2G ) o f a l k a l if e l d s p ar l a t h s , r a r e p l ag i o c la s e a n d i r r e g u l a r i n t e r s t i t i a l q n a r t z . S p h e n e a n d i r o n - o r em i e r o p h e n o c r y s t s a ls o o c c u r a n d g l as s m a y a ls o b e f o u n d i n t e r ,t t i ti a l l y i n t h eg r o u n d m a s s .T h e p h e n o e r y s t s o f s a n i d in e ( 3 A ) a re s m a l l e r i n 3 R t h a n i n 2 R , a n d o c c u r a s i n t e r -l o c k i n g c r y s t a l c l u s t e r s w i t h p l a g i o c l a s e ( 3 B ) b u t p r e d o m i n a n t l y a s i n d e p e n d e n tc r y s ta l s . A u g i t e , a n d b i o t i t e w i t h o n l y a n i n d e f i n i te r i m o f g r a n u l a r m a g n e t i t e ,a r e t h e p r i n c i p a l f e rr o m a g n e s i a n p h e n o c r y s t s, t h e p a l e g r e e n p y r o x e n e c o n t a i n i n gm i c r o p h e n o c r y s t s o f i r o n -o r e . S p h e n e a n d i r o n o r e a r e c o m m o n m i c r o p h e n o c r y s t s.T h e g r o u n d m a s s ( 3 G ) i s v e r y f i n e g ra i n e d w i t h l a t h s o f a lk a l i fe l d s pa r , i n t e r s t i t i a lg l a s s , a s i l i c a min e ra l , ~ n d g ra n u le s o f p y ro x e n e a n d i ro n o re .T h e b i o t i t e p h e n o c r y s t s i n 4 R a r e a l m o s t c o m p l e t e l y r e s o r b e d a n d r e p l a c e d b ya g g r e g a t e s o f m i n u t e i r o n - o r e g r a i n s , a n d t o g e t h e r w i t h p h e n o e r y s t s o f p e r t h i t i ca lk a l i f e ld s p a r (4 A), p l a g io e la se (4 B ) a n d fe r ro a u g i t e a re e n c lo s e d b y a f in e -g ra in e dg r n u n d m a s s (4 G ). A f e w o f t h e a l k a l i fe l d s p ar p h e n o c r y s t s h a v e c o r es o f p a r t i a l l y

r e s o r b e d p l a g i o e l a s e , a n d t h e p y r o x e n e p h e n o c r y s t s i n v a r i a b l y h a v e a n o u t e rr i m o f g r a n u l a r ir o n - o r e. T h e g r o u n d m a s s is m a d e u p o f a l k a l i f e l d sp a r , e r i s to b a l i t e( C a r s t e n s , 1 9 6 1 ) p y r o x e n e , i r o n o r e , b r o w n i s h a m p h i b o l e , a n d r a r e i n t e r s t i t i a lglass .T h e s u b -a c id p i t c h s to n e 1 0 ]g h a s b e e n d e s c r ib e d e l s e wh e re (C a rmic h a e l , 1 9 6 3 a )a n d t h e p u b l i s h e d d a t a f o r t h i s r o c k h a v e b e e n i n c l u d e d i n t h e t a b l e s a n d d i a g r am stb r c o mp le te n e s s .

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T R AC HYT E S AND T HE I R F E L D S P A R P HE NOC R VS T S ] 5 5The uadersaturated trachyte~. P h e n o c t T s t s o f a l k a li f e l d s p a r ( 5A ) , p l a g io c l a s e( 5 B ) , b i o t i te , a u g i t e , o l i v in e , a n d i r o n o r e a r e f o u n d i n 5 R . T h e a l k a l i f e l d s p a r h a s

a n u n u s u a l l y p a t c h y e x t i n c t io n , a n d b o t h t h e z o n e d p l a g io c l as e a n d a l k al i f e ld s p a rp h e n o c r y s t s a r e m a n t l e d b y a t h i n z o n e o f a l k al i f e ld s p a r c r o w d e d w i t h i r o n -o r ei n c lu s i o n s . T h e b i o t i t e is s t r o n g l y p l e o c h r o i c w i t h a t h i n r i m o f g r a n u l a r i r o n - o r ea r o u n d e a c h c r y s t a l . T h e g r o u n d m a s s ( SG ) i s m a d e u p o f f in e l a t h s o f a l k a li f e l d s p a r ,g r a n u l e s o f p y r o x e n e a n d i r o n - o re , a n d a r a r e i n t e r s t i t i a l f e l d s p a t h o i d , p o s s i b l ys o d a l i t c .S a n i d i n e p h e n o c r y s t s (6 A ) m a y o c c u r a s in d e p e n d e n t c r y s t a l s o r e n c lo s e n o r -m a l l y i n d e p e n d e n t pl a g io c l as e p h e n o c r y s t s (6 B ) i n 6 R , a n d t h e p h e n o c r y s t s o fb i o t i te a r e a l m o s t c o m p l e t e l y re s o r b e d a n d r e p l a c e d b y a g g r e g a t e s o f i r on - o re ,w h i c h h a v e in t u r n b e e n d i s i n t e g r a t e d a n d s t r e w n t h r o u g h o u t t h e g r o u n d m a s s .B a r k e v i k i t e p h e n o c r y s t s h a v e b e e n p a r t ia l l y r e s o r b e d a n d r e p l a c e d b y i r o n -o r e ,a n d t h e s p h e n e m i c r o p h e n o c r y s t s a ls o h a v e a r i m o f g r a n u l a r i r o n - o re . P a l e g r e e n is h -y e l lo w a u g i t e a n d r a r e o l i v in e p h e n o c r y s t s a r e u n a f f e c te d b y t h e g r o u n d m a s s , w h i c hi s c r y s t a l li n e a n d f u l l o f i r o n - o r e g r a i n s f r o m t h e d i s i n t e g r a t i o n o f t h e b i o t i t e i r o n -o r e a g g r e g a t e s . T h e g r o u n d m a s s is c o m p o s e d o f a l k a l i f e l d s p a r , p y r o x e n e , a n da p a t i t e w h i c h i s u n u s u a l l y a b u n d a n t .T h e t r a c h y t e 7 R h a s a b u n d a n t p h e n o c l T s t s o f s a n i d in e ( 7A ) s h o w i n g al l g r a d a -t i o n s i n s iz e t o t h e s a n i d in e o f t h e g r o u n d - m a s s . T h e l a r g e r p h e n o c r y s t s m a y f o r mi n t e r l o c k i n g c l u s t e r s o f c r y s t a l s , a f e w o f w h i c h h a v e c o r e s o f p l a g i o c la s e ( A n2 7).S p a r s e p h e n o c r y s t s o f f e rr o a u g i te , s p h e n e , z i r co n , a n d i r o n -o r e t o g e t h e r w i t hs a n i d i n e a r e e n c l o s e d b y a t r a c h y t i c g r o u n d m a s s o f s a n i d i n e ( O ra6 .~ ) i n t e r s t i t i a lg l as s , a n d g r a n u l a r p y r o x e n e a n d i r o n o r e ; t h e r e i s a l i t tl e i n t e r s t i t i a l f e l d s p a t h o i di n t h e g r o u n d m a s s .8 R i s a s a n i d in i t e c o m p o s e d p r e d o m i n a n t l y o f i n t e r lo c k i n g s a n i d i n e c r y s t a ls( 8A ) o f v a r y i n g s i z e, t o g e t h e r w i t h s o d i c f e r r o a u g i t e , s p h e n e , a n d i r o n - o re . P a r t i a l l yr e s o r b e d o l i go c l a se i s r a r e l y f o u n d a s c o r e s t o t h e s a n i d i n e c r y s t a l s , a n d n e p h e l i n ea l t e r e d t o z e o l i te s i s p r e s e n t in t e r s t i t i a l l y t o t h e s a n i d i n e . A p a t i t e , z i r c o n , s o d a l it e ,a n d a h a s t i n g s i t i c a m p h i b o l e a r e a c c e s s o ry .

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